The present invention relates generally to a novel method of seismic exploration, and more particularly, to a method for quantitatively distinguishing among seismic events and for inferring changes in the geological character of the subterranean formation. Seismic data including a plurality of seismic signals or "traces" are acquired with sets of seismic sources and seismic receivers. A measure of the reflection coefficient is obtained for selected seismic events within the seismic data. Attributes quantitatively descriptive of variations in the seismic signal amplitude, as a function of incident angle, are developed from the reflection coefficients for selected seismic events. Predetermined combinations of such attributes can provide the geophysicist with an extremely accurate diagnostic tool for quantitatively distinguishing among seismic events and for inferring changes in the subterranean formation.
In the continuing search for hydrocarbons contained in the earth's subterranean formations, exploration geophysicists have developed numerous techniques for imparting seismic wave energy into the earth's subterranean formations, recording the returning reflected seismic waves and processing the recorded seismic data to produce seismic signals or traces. Such seismic signals or traces contain a multiplicity of information, e.g., frequency, amplitude, phase, etc., which have been related to formation structure, lithology, or pore fluid content. More recently, geophysicists' interests have turned to visually evaluating high intensity seismic events in the seismic signals or traces, and variations in the seismic signal amplitude as a function of range. Exemplary of such focus are Quay, et al., U.S. Pat. No. 3,899,768; Thompson, et al., U.S. Pat. No. 4,375,090 and Ostrander, U.S. Pat. Nos. 4,316,267 and 4,316,268.
In particular, Ostrander indicates that progressive change in the seismic signal amplitude of a high intensity seismic event, as a function of range, is more likely than not an indicator of the presence of a gas-bearing formation. Specifically, progressive seismic signal amplitude changes, in an increasing or decreasing manner, with increasing range is the criterion for identifying gas-bearing formations. Ostrander also discloses a method for seismic signal enhancement to improve the visual resolution of such progressive changes in seismic signal amplitude as a function of range.
Quay recognizes that lateral variations in the seismic data can be attributed to variations of the lithological character of the subterranean formations. Quay obtained such results by extracting selected seismic parameters from a seismic wave and thereafter visually displaying such seismic parameters upon a seismic trace of such seismic wave. The visual correlation of anomalies in such seismic parameters relative to the structural interpretation based upon the seismic traces yields a scheme for visually interpreting seismic record sections.
Thompson discloses that acoustic characteristics associated with hydrocarbon containing formations can be compared with similar synthetic values.
Although evaluation of bright spots has been used as an indicator of gas-bearing formations, such analysis is still a calculated risk, as evidenced by the significant numbers of such bright spots which are nonproductive when actually drilled.